ASP.NET Core Select Helper 抛出未设置为对象实例的对象引用

Question

我创建了一个 ASP.NET Core Web 应用程序，它使用 Entity Framework Core 从我现有的数据库中读取数据。我想在视图中使用 FacilityNames 列表填充选择控件，以便在我的 FacilityRevenue 表中创建一个新条目。

我在 FacilityRevenue Controller 上的 Create 方法如下：

public IActionResult Create()
    {
       PopulateFacilityDropDownList();
        return View();
    }

    private void PopulateFacilityDropDownList
    (object selectedFacility = null)
    {
        var facilityQuery = from f in _context.FacilityMaster
                            orderby f.FacilityName
                            select f;
        ViewBag.FacilityID = new SelectList(facilityQuery.AsNoTracking(), "FacilityID", "FacilityName", selectedFacility);
    }

我的创建视图包含以下标记：

        <label asp-for="FacilityName" class="col-md-2 control-label"></label>
        <div class="col-md-10">
            <select asp-for="FacilityId" asp-items="ViewBag.FacilityID">
                <option value="">-- Select Facility --</option>
            </select>
            <span asp-validation-for="FacilityName" class="text-danger" />
        </div>

我的 FacilityRevenue 模型如下：

 public partial class FacilityRevenue
{
    public string FacilityName { get; set; }
    public DateTime Date { get; set; }
    public decimal? TotalInvoices { get; set; }
    public decimal? TotalRevenue { get; set; }
    public int RecordId { get; set; }
    public int? FacilityId { get; set; }

    public virtual FacilityMaster Facility { get; set; }
}

堆栈跟踪如下：

System.NullReferenceException: Object reference not set to an instance of an 
object.
at Microsoft.AspNetCore.Mvc.Rendering.MultiSelectList.Eval(Object container, 
String expression)
at Microsoft.AspNetCore.Mvc.Rendering.MultiSelectList.
GetListItemsWithValueField()
at Microsoft.AspNetCore.Mvc.Rendering.MultiSelectList.GetEnumerator()
at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
at Microsoft.AspNetCore.Mvc.ViewFeatures.DefaultHtmlGenerator.
GenerateGroupsAndOptions(String optionLabel, IEnumerable`1 selectList,  
ICollection`1 currentValues)
at 
Microsoft.AspNetCore.Mvc.ViewFeatures.
DefaultHtmlGenerator.GenerateSelect(ViewContext viewContext, ModelExplorer 
modelExplorer, String optionLabel, String expression, IEnumerable`1 
selectList, ICollection`1 currentValues, Boolean allowMultiple, Object 
htmlAttributes)
at 
Microsoft.AspNetCore.Mvc.TagHelpers.SelectTagHelper.Process(TagHelperContext 
context, TagHelperOutput output)
at   
Microsoft.AspNetCore.Razor.TagHelpers.TagHelper.
ProcessAsync(TagHelperContext context, TagHelperOutput output)
at Microsoft.AspNetCore.Razor.Runtime.TagHelpers.TagHelperRunner.
<RunAsync>d__0.MoveNext()

如何纠正上述错误并实现使用工具列表水合选择控件的功能？

Answer 1

问题是以下行中的拼写错误：

ViewBag.FacilityID = new SelectList(facilityQuery.AsNoTracking(), "FacilityID", "FacilityName", selectedFacility);

"FacilityID" 必须是 "FacilityId"。这就是您在模型中定义的内容：

public int? FacilityId { get; set; }